1. Field of the Invention
The present invention relates generally to spent fuel storage and, more particularly, is concerned with a self-latching reactivity-reducing device for allowing placement of a spent fuel assembly in a fuel storage facility.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
At the end of their useful life, spent fuel assemblies are removed from the reactor core and replaced with fresh fuel assemblies. Because of the lack of any permanent off-site spent fuel disposal facility at the present time, nuclear power plant utilities are forced to store all spent fuel assemblies in pools at on-site fuel storage facilities. However, these on-site storage facilities were originally designed to hold only a fraction of the fuel used over the operating life of the plant. Spent fuel storage pool reactivity, K-eff, is limited in most nuclear plants by technical specifications to being less than 0.95. These technical specifications thus limit the ability to increase the number of spent fuel assemblies which can be stored on-site
The spent fuel pool reactivity limit is also at odds with modern nuclear fuel management strategies. Most utilities are increasing the U-235 enrichments of the fuel that is used. This increase allows the utilities to reduce the total number of fuel assemblies they need to buy and store.
There exist several options to increasing storage capability or fuel enrichment limits. These options range from very expensive reracking operations to a criticality safety reanalysis. One of the most effective methods available today is called "burnup credit" analysis. In this form of criticality safety analysis, calculations are performed to show that a fuel assembly can be safely stored after it has accumulated a minimum amount of burnup. The minimum amount of burnup is dependent on the initial U-235 enrichment of the assembly in question. At high enrichments the burnup requirement may be very large. The burnup requirement is often large enough to preclude the storage of a significant number of fuel assemblies.
Each of the problems described above could be overcome by reducing the reactivity of the fuel assemblies to be stored. One technique is to insert neutron absorber or poison rods into the spent fuel assembly to reduce the reactivity of the fuel assembly so that it can be stored in the on-site fuel storage facility. Representative of this technique is the approach disclosed in European Pat. application No. 0,061,043 to Kuhnel et al and French Pat. application No. 2,544,541 to Foussard. Each of these publications discloses the use of a device attachable to the top nozzle of the spent fuel assembly for locking a cluster of poison rods against removal in the fuel assembly.
One drawback of the locking devices disclosed in the cited publications stems from their reliance upon the exercise of some human effort in rendering them effective, and from their relatively easy accessibility at the top of the fuel assembly. If it is too easy to unlock or unfasten the devices and remove the cluster of poison rods, then these arrangements might be considered as removable under current regulatory standards. The current regulatory standards do not allow credit to be taken for removable neutron absorbers. Thus, simply putting conventional neutron absorber or poison rods into a spent fuel assembly and locking them in the fuel assembly at its top nozzle may not be sufficient to meet current standards.
Consequently, a need exists for an improved approach to lowering reactivity in a spend fuel assembly so as to permit its placement in an on-site fuel storage facility.